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Getting started with Java Threads – Part 2 : join() and sleep()

August 11, 2015January 1, 2017

The thread class provides some methods to define the threads behavior.
Two important methods to consider are:
– join(), to make the other instanced threads to wait for it to day. You can provide a timeout as parameter.
– sleep();

The sleep() method throws a checked exception called “InterruptedException”,each time the thread receives an interrupt request. You have to handle in the code by wrapping the method in a try-catch block.

The following code will make you understand the join method.
We define a MyThread thread:

public class MyThread implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}

Then we can observe the behaviour running the following main:

		public static void main(String[] args) {

		// extending Thread
		Thread myThread = new Thread(new MyThread());
		Thread myJoiningThread = new Thread(new MyThread());

		myThread.setName("myThread");
		myJoiningThread.setName("myJoiningThread");

		System.out.println("Threads will be started...");

		myJoiningThread.start();

		try {
			// waits for this thread to die
			myJoiningThread.join();
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
		myThread.start();

	}

Without the try-catch block in which the join method is invoked the threads would be started asynchronously (the order is not predictable) and in the console you might get either:
Threads will be started... Implementing the runnable interface - Thread name: myThread - priority: 5 - group: main Implementing the runnable interface - Thread name: myJoiningThread - priority: 5 - group: main

or:

Threads will be started... Implementing the runnable interface - Thread name: myJoiningThread - priority: 5 - group: main Implementing the runnable interface - Thread name: myThread - priority: 5 - group: main

But if you include the try-catch block, the myJoiningThread will always be invoked first.

To understand the sleep method you can create a MySleepingThread:

public class MySleepingThread implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}

Then you can run the following main method:

public static void main(String[] args) {

		// extending Thread
		Thread myThread = new Thread(new MyThread());
		Thread mySleepingThread = new Thread(new MySleepingThread());
		myThread.setName("myThread");
		mySleepingThread.setName("mySleepingThread");

		System.out.println("Threads will be started...");

		mySleepingThread.start();
		myThread.start();

	}

You can see in the console that the sleeping thread will be started after 3 seconds.

The sleep method is static. By invoking “Thread.sleep(3000)” you are delaying the creation of the current thread.

If you use the method in the main method, the sleeping thread will be “main”.

Getting started with Java Threads – Part 1

August 11, 2015January 1, 2017

I am learning for the Java OCP 7 and summarizing some relevant infos about Multithreading.
For now I am referring to chapter 13 in the book by G.Ganesh.

You can create threads in 2 ways: extending the Thread class or implementing the Runnable interface
Extending the Thread class is more convenient if you don´t need to extend another class that is not “Thread” (since in java you can only extend one class), because you can use the class methods directly (like “getName()“).
If you implement the Runnable interface (preferred way because of inheritance) you will have to use the static method “currentThread()” of the Thread class to do your operations on the thread (like setting or getting the name).

The two fundamental methods to know are:
– run(): like a main method, necessary to enable the execution of a thread.
– start(): that creates the thread.

The thread will be terminated when the run method execution is complete. It´s invoked implicitly by the JVM (DON´T invoke it explicitly!).
The main method starts a Thread called “main“. If you create and start your own thread it will have default names like “Thread-0“, “Thread-1“.

If you extend the thread class you need to override the run() method (otherwise it does nothing). If you implement the Runnable interface you will have to implement the run() method (mandatory)

The following code will help you to understand more about these first concepts.

public class MyThread1 extends Thread {

	public void run() {
		System.out.println("Extending the Thread class - Thread name: "
				+ getName() + " - priority: " + getPriority() + " - group: "
				+ getThreadGroup().getName());

	}

}

public class MyThread2 implements Runnable {

	public void run() {
		Thread thread= Thread.currentThread();
		System.out.println("Implementing the runnable interface"
				+ " - Thread name: " + thread.getName()
				+ " - priority: " + thread.getPriority()
				+ " - group: " + thread.getThreadGroup().getName());

	}

}

public class ThreadsDemo {

	public static void main(String[] args) {

		// extending Thread
		Thread myThread1 = new MyThread1();
		// implementing Runnable
		Thread myThread2 = new Thread(new MyThread2());

		System.out.println("Threads defined. Default properties: ");
		//name, priority, group returned by the toString method 
		System.out.println(myThread1);
		System.out.println(myThread2);
		System.out.println("Changing default names...");
		myThread1.setName("Thread_1");
		myThread2.setName("Thread_2");
		System.out.println("Threads will be started...");
		myThread1.start();
		myThread2.start();

	}

}

The output of the main method should be something like:

Threads defined. Default properties: Thread[Thread-0,5,main] Thread[Thread-1,5,main] Threads will be started... Extending the Thread class - Thread name: Thread-0 - priority: 5 - group: main Implementing the runnable interface - Thread name: Thread-1 - priority: 5 - group: main

Launching the main program you see that the threads are started asynchronously. Sometimes you see:
Implementing the runnable interface - Thread name: Thread-1 Extending the Thread class - Thread name: Thread-0

Sometimes it´s:
Extending the Thread class - Thread name: Thread-0 Implementing the runnable interface - Thread name: Thread-1

You can quickly proof it in Eclipse launching the program with the shortcut Ctlr+11.

To change this “random” behaviour you can set the priority with the setPriority() method, from 1, the lowest, to 10, the highest. The default is 5. They can be all retrieved by the static members:
– Thread.NORM_PRIORITY ;
– Thread.MAX_PRIORITY ;
– Thread.MIN_PRIORITY .

Threads can be in one of the following states:
– NEW (created)
– RUNNABLE (started)
– TERMINATED
– BLOCKED (waiting to acquire the lock)
– WAITING (waiting for notifications)
– TIMED_WAITING (sleep() invoked and the thread is sleeping or if wait() with timeout)

States are defined with the Thread.State enumeration.
You can get the state with the getState() method.

Convert audiofiles on Ubuntu by the command line with soundconverter

August 9, 2015January 1, 2017

Linux , Programming , shell

If you need to convert audio files in another format you can use the tool called “soundconverter”, that you can simply install:

~$ sudo apt-get install soundconverter

Then simply run a command like:

~$ soundconverter -b -m "audio/mpeg" -s ".mp3" /home/laura/cd-wav/*.wav

“-b” stands for batch type

“-m” to specify the mime format.

“-s” to specify the suffix

And move them into another folder eventually:

~$ mv /home/laura/cd-irene/*.mp3 /home/laura/cd-wav.

For more info check the official wiki pages:
http://wiki.ubuntuusers.de/soundconverter
http://manpages.ubuntu.com/manpages/karmic/man1/soundconverter.1.html

Wildfly Apache Modcluster : Error MODCLUSTER000042

May 27, 2015January 1, 2017

java , Programming

I was dealing with the Apache modcluster configuration for the Wildfly load balancing and I ran into the following error

[Server:server-four] 16:55:00,093 ERROR [org.jboss.modcluster] (UndertowEventHandlerAdapter - 1) MODCLUSTER000042: Error MEM sending STATUS command to localhost/127.0.0.1:6666, configuration will be reset: MEM: Can't read node

The error occurs when the servers are “idle”, that is where there is nore request from the browser for a time longer than the keep alive timeout setting.

PL/SQL Transactional single rows locking with cursors

May 22, 2015January 1, 2017

Programming , Script , SQL

Oracle already provides an automatic exclusive locking mechanism for the rows you want to update. However you can override it to customize the performance (for example, if you need consistent data and/or exclusive access for the duration of a whole and more complex transaction).

When your application requires consistent data for the duration of the transaction, not reflecting changes by other transactions, you can achieve transaction-level read consistency by using explicit locking, read-only transactions, serializable transactions, or by overriding default locking.

Row locking at transactional level can be achieved with “SELECT FOR UPDATE” statement. The lock is released after a commit or rollback only.

If you want to lock single rows in Oracle PL/SQL you can use cursors, pointing at the rows you want to lock.

The following example show you how to lock single rows:

CREATE OR REPLACE PROCEDURE LOCK_ORDER_ENTRY 
(id_number IN number, system_user IN varchar2)
IS
/*"order_row" is a variable to store the row found by the cursor select statement   */
order_row order_queue%rowtype;
cursor c1 is
    SELECT * 
    FROM order_queue 
    WHERE id=id_number
    FOR UPDATE NOWAIT;
BEGIN
/* first of all you need to open the cursor */
OPEN c1;
/* then you need to fetch the content in the variable */
   LOOP
      FETCH c1 INTO order_row;
/* the lock will be released after the commit */
      IF (c1%found) THEN
         UPDATE order_queue SET processed=1, user=system_user where CURRENT OF c1;
         COMMIT; 
      END IF;
 /* then you need close the cursor */
  END LOOP;
CLOSE c1;
END LOCK_ORDER_ENTRY;

The “select for update” statement has two possible variants
– FOR UPDATE NO WAIT, that triggers an error if the row is locked by another user.
– FOR UPDATE SKIP LOCKED, that fastens the execution by skipping the already locked rows

If you need to lock a single row in the database you don´t need a loop.

Install Apache web server on Windows

May 8, 2015January 1, 2017

Programming , Software development

To install apache on Windows, you can download the binary from:
http://www.apachehaus.com/cgi-bin/download.plx?dli=gUEJEMjNVWx4EVV9yUsZVTJVlUGRVYSFlUuB3T

Then you need to edit the httpd.conf file to change the path to the server root (SRVROOT). It should be something like:
Define SRVROOT “C:\Users\laura\httpd-2.4.12-x86-r2\Apache24”

If you are running Skype (that listens on the port 80), there will be a conflict In this case you need either to kill Skype or change the default Apache listening port in the httpd.conf

If you need to enable apache modules (if no yet enabled) that you might need for your activities you need the “a2enmod” command.

There are several solutions that you can fidn by googling, like:
http://aninternetpresence.net/program/
https://github.com/enderandpeter/win-a2enmod

Wildfly Standalone clustering

May 1, 2015January 1, 2017

java , Programming , Software development

To set up a cluster of standalone servers you have two possibilites: nodes running on different machine (horizontal scaling) or running on the same machine (vertical scaling).

To make a wildfly standalone cluster with 2 nodes on the same machine you can follow the steps below:

1) copy the standalone folder and rename it to „standalone1“, „standalone2“

2) create 2 scripts in the /bin folder, to avoid typing all the parameter in the command line each time.

script1.sh

#!/bin/sh
./standalone.sh -Djboss.server.base.dir=$JBOSS_HOME/standalone1 -Djboss.node.name=server1 -c standalone-full-ha.xml

script2.sh

#!/bin/sh
./standalone.sh -Djboss.server.base.dir=$JBOSS_HOME/standalone2 -Djboss.node.name=server2 -c standalone-full-ha.xml -Djboss.socket.binding.port-offset=100

To bind the public interface to a specific address add the IP as parameter like the following :
-b 192.168.1.1
For the management console it´s something like:
-bmanagement=192.168.1.1

To bind to all available IPs set them to „0.0.0.0“ like:
-b 0.0.0.0 -bmanagement=192.168.1.1

The high availability configuration has default multicast addresses set to 230.0.0.4.

They can also be changed in the socket-binding section in the standalone xml file.

Alternatively you can add it as parameter like : -u=230.0.1.2

The port offset must be considered, if you need to run the cli for the second instance specify the port:
./jboss-cli.sh --controller=localhost:10090 --connect

3) Set the hornetQ clustering username and password.

The cluster nodes must have the same user und password otherwise you will get:

ERROR [org.hornetq.core.server] (default I/O-1) HQ224018: Failed to create session:
HornetQClusterSecurityException[errorType=CLUSTER_SECURITY_EXCE
PTION message=HQ119099: Unable to authenticate cluster user: HORNETQ.CLUSTER.ADMIN.USER]

To fix this add the cluster username and password in the subsystem messaging:2.0 for any standalone xml configuration file under the tag hornet1-server :

 <clustered>true</clustered>
<cluster-user>clusteruser</cluster-user>
<cluster-password>cluster-secret</cluster-password>

Otherwise you can set to false if you don ́t need to cluster the messaging.

4) run the scripts in two different terminals (start with server 1).

5) deploy the app.
The clustering service will be initiated only if a cluster-enabled application is deployed.
In the web.xml of the application you need to add the distributable tag:

<distributable/>

You need to deploy the app in every server instance

You can try it with the app by Arun Gupta called „clustering“, that you can find at https://github.com/arun-gupta/wildfly-samples.
To make it a little simpler you can create a maven simple project and copy the content in the webapp folder (https://github.com/arun-gupta/wildfly-samples/tree/master/clustering/http/src/main/webapp).
Then you can edit the pom.xml like the following:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>de.demo</groupId>
  <artifactId>demo-clustered-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>war</packaging>
  <name>Demo_Clustered_App</name>

  <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>javax</groupId>
            <artifactId>javaee-api</artifactId>
            <version>7.0</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <failOnMissingWebXml>false</failOnMissingWebXml>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Run mvn package. In the target folder you will find the .war file, that you can manually deploy in each management console.
At the following urls you will see that the node server names are different for the two urls but the session data is shared:
http://localhost:8080/demo-clustered-app-0.0.1-SNAPSHOT/index.jsp
http://localhost:8180/demo-clustered-app-0.0.1-SNAPSHOT/index.jsp

Once you get your app deployed on both instances in the logging of the server one you will see something like:

INFO [org.infinispan.remoting.transport.jgroups.JGroupsTransport] (Incoming-1,shared=udp) ISPN000094: Received new cluster view: [server1/web|1] (2) [server1/web, server2/web]

Defining a port offset is not the only way to set up a cluster on the same machine.
You can also define multiple IP addresses on the same machine (multihoming).

Ubuntu 14.04 : launchpad.net NO_PUBKEY issue when running apt-get update

November 14, 2014

Linux , Programming , shell

Launching sudo apt-get update you might get the NO_PUBKEY issue. Something like:

W: GPG-Fehler: http://ppa.launchpad.net trusty Release: Die folgenden Signaturen konnten nicht überprüft werden, weil ihr öffentlicher Schlüssel nicht verfügbar ist: NO_PUBKEY EFD5FA852F20733F

Well it´s very simple. Just execute the following (specifying the missing public key):

laura@FFM:~$ sudo apt-key adv --keyserver.ubuntu.com --recv-keys EFD5FA852F20733F

And then run sudo apt-get update again. The error will disappear.

Hibernate inheritance: Discriminator Column vs Formula

November 12, 2014January 1, 2017

Programming

Java allows you to take advantage of the domain model and think in terms of objects to elaborate technical solutions to real world problems like transferring money, buying products online and so on. These domain objects are expected to be stored and retrieved from a database, but storing the domain objects into a relational database is not so straightforward. The object-relational paradigm is mismatching, because the objects in our application are no table in a row-column format.
Hibernate allows to handle the object-relational mismatch through ihneritance strategies. The default one is the single table strategy. To achieve it you need to use discriminators.
You might add discriminator columns, but most times it´s not a good choice, especially for legacy systems, because it takes an extra column (called “d_type” as default name). So you might consider better solutions, like using the discriminator formula instead.

DiscriminatorFormula is an alternative to DiscriminatorColumn.
With DiscriminatorColumn it creates an additional column (as default called “dtype”) which contains the discriminator value.
You put the annotation in the superclass, (which maps the real table as default):

    @Entity
    @Table(name = "features")
    @DiscriminatorColumn
     public class Features{
    //valid code
    }

DiscriminatorFormula allows you to check the database row content and “select” a subclass by the discriminator value. No additional (“dtype”) column is created.
In the main class you annotate the superclass with the formula, like:

    @Entity
    @Table(name = "features")
    @DiscriminatorFormula(
    		"CASE WHEN num_value IS NOT NULL THEN 'NUMERIC' " +
	   		" WHEN txt_value IS NOT NULL THEN 'TEXT' end")
     }
     public class Features{
    //valid code
    }

In the DiscriminatorFormula you just put some pure SQL to do what you need.

You can choose one of these two options and the subclasses is exactly the same in both cases.
the subclasses are annotated java object (with @Entity), but no real table, they are just useful to map columns in the table represented by the main class.
In the subclasses you specify the discriminator value, for Example:

    @Entity
    @DiscriminatorValue('NUMERIC')
    public class NumericFeatures extends Features{

    private Double numValue;

	public Double getNumValue() {
		return numValue;
	}

	public void setNumValue(Double numValue) {
		this.numValue = numValue;
	}
   //valid code

}

In the table called “features” you have both columns “num_value” and “txt_value”, containing the corrensponding values.
With DiscriminatorColumn you would have either “NUMERIC” or “TEXT” value in the dtype column and also both the “num_value” and “txt_value” columns.

If you don´t specify the inheritance strategy the default is “SINGLE_TABLE”. the following annotation can be omitted if it´s your chosen strategy:

     @Inheritance(strategy = InheritanceType.SINGLE_TABLE)

With or without this annotation you get one table called “Features”.

MySQL CHECK constraint alternative: triggers!

November 12, 2014January 1, 2017

java , Programming , Script , Software development , SQL

Yesterday I discovered the powerful hibernate check constraint (@org.hibernate.annotations.Check) , which can be directly added in your entity class like this:

@Entity
@XmlRootElement
@Check(constraints = "age IS NOT NULL")
public class Person{

String firstname;
String lastName;
Integer age;

// valid code
}

Unfortunately, as you can read in the official MySQL docs, “the CHECK clause is parsed but ignored by all storage engines”.
Yesterday I found out that MySql 5.X doesn´t support the SQL CHECK constraint.
It means that if you are using JPA und HIBERNATE you can´t take advantage of the Check annotation!

In the project I am working on we could successfully export the schema with the maven command “hibernate4:export”. The check constraint was added in the create table statement. So if your DBMS supports it you get the job done in a very elegant way.

The way out I could find by googling a bit is not so elegant, but it allowed me to achieve the same result. I have just written a trigger, like the (simple) following:

CREATE TRIGGER check_age
BEFORE INSERT ON person
FOR EACH ROW
BEGIN
IF age is NULL
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Age cannot be null';
END IF;
END;

MySQL has been bought by Oracle. Is this silent ignoring is a strategy to make people migrate to Oracle DBMS? I am afraid it´s so. Corporation games, my friends!

Category: Programming

Getting started with Java Threads – Part 2 : join() and sleep()

Getting started with Java Threads – Part 1

Convert audiofiles on Ubuntu by the command line with soundconverter

Wildfly Apache Modcluster : Error MODCLUSTER000042

PL/SQL Transactional single rows locking with cursors

Install Apache web server on Windows

Wildfly Standalone clustering

Ubuntu 14.04 : launchpad.net NO_PUBKEY issue when running apt-get update

Hibernate inheritance: Discriminator Column vs Formula

MySQL CHECK constraint alternative: triggers!

Linux Day 2011 Call for Papers (IT)

Discovering Sqlite by Banshee media player

First steps in Sqlite and Perl

Sqlite database backup: the .dump command

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